1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device having a color filter on thin film transistor structure and a method for fabricating the same.
2. Description of the Related Art
In general, a liquid crystal display device (LCD) displays images using the optical anisotropy and double refraction properties of liquid crystal molecules. The arrangement of the liquid crystal molecules is changed by an applied electric field. The light transmittance of the liquid crystal molecules also changes in accordance with the alignment direction of the liquid crystal molecules.
The LCD device includes two substrates facing each other. Electrodes are provided on the facing surfaces of the respective substrates for generating an electric field. A liquid crystal material is injected between the two substrates. The alignment direction of liquid crystal molecules is changed by the electric field generated by a voltage applied to the two electrodes. Thus, the LCD device displays an image by varying the light transmittance of the liquid crystal molecules in accordance with the alignment direction of the liquid crystal molecules.
FIG. 1 is a schematic plane view of the related art LCD device. Referring to FIG. 1, the related art LCD device 11 includes an upper substrate (not shown) including a color filter (not shown) and a common electrode (not shown) deposited on the color filter (not shown). The color filter (not shown) includes sub-color filters (not shown) and a black matrix (not shown) formed between the sub-color filters (not shown). A liquid crystal material (not shown) is filled between the upper substrate (not shown) and the lower substrate (not shown). The lower substrate is also called an array substrate.
Pixel regions P are defined on a lower substrate (not shown) of the LCD device. A pixel electrode (not shown) and a switching device T are formed at each pixel region. Gate lines 13 and data lines 15 lines crossing each other form an array. Crossings of the gate lines and data lines define the pixel regions P. The switching devices T, for example thin film transistors (TFT), are arranged in a matrix on the lower substrate. Each of the switching devices is electrically connected to one of the gate lines 13 and one of the data lines 15.
A transparent pixel electrode 17 is formed at each of the pixel regions P. The pixel electrode 17 is formed of transparent conductive metal having a relatively high light transmittance, such as indium-tin-oxide (ITO). Storage capacitors C are formed on the gate line 13. The storage capacitors C are electrically connected to the pixel electrodes 17 in parallel. A part of the gate line 13 is used as a first electrode of the storage capacitor C, and a source/drain metal layer 30 having an island shape and formed of the same material as source/drain electrodes of the switching device T is used as a second electrode of the storage capacitor C. The source/drain metal layer 30 contacts the pixel electrode 17 to receive signals from the pixel electrode 17.
As described above, when the liquid crystal panel is formed by attaching the upper color filter substrate (not shown) and the lower array substrate (not shown), light leakage or other problems may occur because of misalignment between the color filter substrate (not shown) and the array substrate (not shown).
FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1 illustrating a method of fabricating the related art LCD device. Referring to FIG. 2, the method for fabricating the related art LCD device includes disposing an array substrate as a lower substrate 22, and a color substrate as an upper substrate 5, with a specific gap therebetween; and injecting liquid crystals 14 between the lower and upper substrates 22 and 5. Also, switching devices T (shown in FIG. 1), for example thin film transistors, and a passivation layer 40 are formed on the lower substrate 22. Each of the thin film transistors includes a gate electrode 32, an active layer 34, a source electrode 36 and a drain electrode 38. The passivation layer 40 protects the thin film transistors formed on the lower substrate 22.
A transparent pixel electrode 17 is formed at each pixel region P. The transparent pixel electrode 17 contacts the drain electrode 38 of the switching device T. Storage capacitors C (shown in FIG. 1) are formed on a gate line 13 to be electrically connected to the pixel electrodes 17 in parallel. In the related art array substrate, the data line 15 and the pixel electrode 17 are separated from each other by a specific interval A to prevent vertical cross talk. The gate line 13 and the pixel electrode 17 are also separated from each other by a specific interval B.
Red, green and blue color filters 8a, 8b and 8c are formed on the upper substrate 5 to correspond to the pixel regions P of the lower substrate 22. A black matrix 6 is also formed on the upper substrate 5 to correspond to the gate lines 13, the data lines 15 and the switching devices T. The black matrix 6 formed at the upper substrate 5 covers the gaps A and B between the data line 15 and the pixel electrode 17, and between the pixel electrode 17 and the gate line 13, to block light leaking through the gaps A and B. Also, the black matrix 6 overlies the thin film transistor T to block irradiated external light from passing through the passivation layer 40 and affecting the active layer 34.
The upper substrate 5 and the lower substrate 22 may be misaligned during the attachment process. In consideration of such misalignment, a specific margin is included when the black matrix 6 is designed. The margin causes a corresponding decrease in an aperture ratio. If the misalignment error exceeds the margin, light leakage regions A and B may not be completely covered by the black matrix 6. Thus, light leakage occurs in these regions. Accordingly, image quality deteriorates.
As described above, the related art LCD device employs a method of fabricating a color filter substrate and a thin film transistor array substrate through different processes and attaching them together. Recently, a new design concept for a thin film transistor array, called a Color Filter on TFT (COT) method in which a color filter is formed on a thin film transistor array substrate, has been introduced. The LCD device employing the COT method is fabricated in such a manner that the switching devices, for example TFTs, are formed, and then, red, green and blue color resins are formed on the TFTs.
FIG. 3 is a schematic plane view of an LCD device having a COT structure in accordance with the related art. Referring to FIG. 3, gate lines 102 and data lines 116 cross each other. A switching device T, including a gate electrode 104, an active layer 108 and source/drain electrodes 112 and 114, is formed at each crossing of these gate and data lines 102 and 116. Also, transparent electrodes (not shown) contacting the drain electrodes 114 and colors filter 124a, 124b and 124c are formed at regions defined by crossings of the gate and data lines 102 and 116. The transparent electrodes (not shown) are formed on the color filters 124a, 124b and 124c. The color filters indirectly contact the drain electrodes 114 through the transparent electrodes (not shown). Also, each of the transparent electrodes (not shown) is electrically connected to the storage capacitor C formed on the gate line 102. The storage capacitor C uses a part of the gate line 102 as a first electrode, and uses a capacitor upper electrode 118 as a second electrode. The capacitor upper electrode 118 is electrically connected to the transparent electrode (not shown) and is concurrently formed on the same layer as the source/drain electrodes 112 and 114.
In accordance with the COT structure, a black matrix 120 and the red, green and blue color filters 124a, 124b and 124c are formed on the switching device T of the array part. The black matrix 120 covers regions where light might leak. The black matrix 120 is formed by applying an opaque material, blocks light, and protects the switching device T.
FIGS. 4A to 4E are cross-sectional views taken along line IV-IV of FIG. 3 illustrating a method for fabricating an LCD device having a COT structure in accordance with the related art. Referring to FIG. 4A, a conductive material is deposited on a substrate 100. The deposited conductive material is patterned to form a gate line 102 and a gate electrode 104. Then, a gate insulation film 106, which is a first insulating film, is formed by depositing an inorganic insulating material, for example silicon nitride (SiNx) or silicon oxide (SiO2), over the entire surface of the substrate 100, including the gate line 102 and the gate electrode 104 formed thereon. Then, an active layer 108 and an ohmic contact layer 110 are formed on the gate insulation film 106 by depositing, then patterning, an intrinsic amorphous silicon (a-Si:H) and an impurity-doped amorphous silicon (n+a-Si:H) on the gate insulation film 106.
Then, a conductive metal, such as chrome (Cr), molybdenum (Mo), copper (Cu), tungsten (W), tantalum (Ta) and the like, is deposited over the entire surface of the substrate 100, including the active layer 108 and the ohmic contact layer 110 thereon. The deposited conductive metal is patterned to form a source electrode 112 and a drain electrode 114, a data line 116, and a capacitor upper electrode 118. The source electrode 112 and the drain electrode 114 contact, respectively, the ohmic contact layer 110. The data line 116 contacts the source electrode 112. The capacitor upper electrode 118 is a storage node formed on the gate line 102 and has an island shape.
Then, a second insulating film 119 is formed by depositing an inorganic insulating material, such as silicon nitride and silicon oxide, over the entire surface of the substrate 100, including the source and drain electrodes 112 and 114 thereon. The second insulating film 119 prevents a potential defective contact between the active layer 108 and an organic film (not shown) to be formed hereafter. The second insulating layer 119 is not formed if the contact is not defective.
Then, a black matrix 120 is formed over the switching device T, the data line 116 and the gate line 102 by depositing an opaque organic material on the second insulating film 119 to form an organic layer, and patterning the organic layer. In an embodiment of the present invention, a transparent organic insulating material or an inorganic insulating material having a low permittivity may be used as a passivation film for protecting the switching device T, instead of the black matrix 120. In this case, a special black matrix may be used at an upper substrate of the LCD device.
Referring to FIG. 4B, the black matrix 120 is selectively patterned. Portions of the black matrix 120 are removed at a region corresponding to a contact hole to be formed for contacting a drain electrode, at a region where the capacitor upper electrode 118 electrically contacts a common electrode. The remaining portions of the black matrix 120 overlap the thin film transistor T region and the storage capacitor C region. Then, a color resin is applied to an upper surface of the entire structure including the selectively-patterned black matrix 120 to form red, green and blue color filters 124a, 124b and 124c in a plurality of pixel regions.
Referring to FIG. 4C, an acryl resin is applied to an upper surface of the entire structure including the color filters 124a, 124b and 124c to form an overcoat layer 126.
Referring to FIG. 4D, the overcoat layer 126 and the black matrix 120 are selectively patterned to form a drain contact hole 128 and a capacitor contact hole 130 exposing parts of the drain electrode 114 and the capacitor upper electrode 118.
Referring to FIG. 4E, a transparent electrode material is deposited on the overcoat layer 126 including the drain contact hole 128 and the capacitor contact hole 130. The transparent electrode material is patterned to form a common electrode 132.
In the related art array substrate of the LCD device, the overcoat layer, such as an acryl resin, is used on the TFT lower substrate for the COT structure to prevent a decrease in a aperture ratio caused by a corresponding increase in an attachment margin when manufacturing a large glass substrate. The acryl film flattens an uneven surface generated by the organic film of the lower substrate and prevents a flow of impurity ions from the color filters to a liquid crystal layer. However, the use of the acryl material increases cost. Furthermore, although temporarily improved through a post exposure process, the transmittance of the acryl keeps decreasing during subsequent processes, thereby reducing the transmittance of the LCD panel. Therefore, to avoid such defects of the COT structure, research is actively ongoing on a COT structure that does not include acryl.